Tonight at professor we had a mage discover that if you use invisibility right at the phase change you could avoid getting into the tear gas leaving you uneffected.
This seemed to work for rogues vanishing and hunters FD'ing too, so im guessing it has something to do with leaving combat. This is especially usefull doing the change from P2-P3 as it allows atleast a few dps from your raid to burn him down while he's drinking potions from his table.
As this is most likely not intended by blizzard i think we should expect it to be hotfixed soon.
I hope you found this as usefull as we did.
//Cowgoesmoo